Atmospheric carbon dioxide (CO2) concentration being from pre-industrial 270 ppm to the current 380 ppm, CO2 emission mitigation has become the focus of global concern. The government has attached great importance to the issue; all walks of life are working hard to explore the new technologies of CO2 emission mitigation, especially in the wastewater treatment process.
Microalgae and cyanobacteria (blue-green algae) can convert H2O and CO2 into organic compounds by photosynthesis using photon energy, and has the growth and reproduction capabilities by using mineral nutrients and some organic compounds which are absorbed from the environment. Therefore, it can be used for absorption of wastewater nutrients or biological fixation of CO2 emissions. Its reproductive biomass produced by algae can be used as material for bio-fuels (for example, bio-diesel, ethanol or methane), animal feed additives, organic fertilizers, etc. Technically, microalgae can be used for many purposes, but the economic feasibility is determinant. Therefore, the cultivation technology, separation technology, and follow-up utilization technologies will greatly affect the utilization and economic viability of microalgae, wherein, high-density cultivation of microalgae by using organic wastewater is a key for wastewater utilization, carbon mitigation for wastewater treatment and low cost microalgae cultivation. And to build photobioreactor (microalgae cultivation reactor) which has a high solar energy utilization rate and high carbon fixation rate suitable for microalgae growth, is the key technology for high-density cultivation of microalgae, wastewater utilization and carbon mitigation for organic wastewater treatment, and has great influence to product quality and production costs of microalgae.
1) Open Photobioreactor
The open photobioreactor is one of the most widely used and mature photobioreactor in microalgae cultivation. It includes two basic types—the horizontal type and oblique type (AE Richmond & Soeder, 1986). Simple structure and low cost are its outstanding advantages. The most typical and commonly used open pond culture system is track pool reactor designed by Oswald (1969), where the mixture and circulation flow of algae liquid are realized by the rotation of the paddle wheel and whirling arm. Now it has enlarged to 180,000 m2 in Taiwan Province of China (chlorella production) and 200,000 m2 in Mexico (Spirulina production). Since the development of open culture systems, its overall structure is still without a big change. As the open photobioreactor has disadvantages—such as lack of temperature and light control, inadequate mixing, medium susceptible to being contaminated by air-borne micro-organisms and dust. So that there is instability in culture conditions, low photosynthetic efficiency and carbon sequestration efficiency, low overall productivity (the concentration of microalgae in general is 0.1-0.5 g/L or so); and narrow scope of application (only applicable to few microalgae species such as Spirulina, Chlorella, Dunaliella salina). The majority of scholars generally believe that the open culture system technology has been developed to the limit, and how to meet the needs of the rapid development of microalgae biotechnology, and to develop a new photobioreactor have become a long-term goal.
2) Closed Photobioreactor
The shortcomings of open channel promote the development of a closed system. The radical solution to the high-density microalgae cultivation is to develop closed photobioreactor, which is a photobioreactor made of transparent tube or container (A Richmond, 1990; Tredici, 2004). In 1950, Cook developed the first vertical tubular photobioreactor. Now, the closed photobioreactor have various types and forms, mainly three types of photobioreactor, namely tubular, plate and traditional tank.
A. Tubular Photobioreactor
Tubular photobioreactor generally uses the transparent hard material (glass or plastic) bent into different shapes with small diameter and connect together, which contains horizontal placed serpentine tubular reactor, double-layer arrangement tubular reactor, multi-branch parallel flow tube reactor, a-type tubular reactor, circular spiral coiled tubular reactor and so on. Tubular photobioreactor is usually with a gas exchange zone, for adding CO2 and nutrients, and removing the oxygen from return flow. The zone is connected with the two pipe ends respectively using the pump or the air-lift device to make the medium circulate in ducts and pipeline (Pirt et al., 1983; A Richmond, Boussiba, Vonshak, & Kopel, 1993).
Most of the existing tubular photobioreactors have the following problems: {circle around (1)} There are difficulties in gas exchange addition of carbon and release of oxygen in the tube cannot be done in time. The dissolved oxygen level of most tubular photobioreactors is more than 200%, so that the photosynthesis and cell growth are greatly inhibited. {circle around (2)} Serious algal cell damage is easily caused because of large shear stress when using mechanical pumps to circulate the algae fluid (Gudin & Chaumont, 1991). Large-scale mixing is even more uneven and mass transfer efficiency is poor. {circle around (3)} A plug flow state easily forms in the thin and long straight tube, and there are difficulties in forming a good mixing in the cross-section. Due to shading of light by microalgae and limited light path length, microalgae in the reactor cannot receive photon adequately, and the light conversion efficiency of the reactor is low; {circle around (4)} It is difficult to clean once there is attached-growth of the algae on the internal wall of reactor; {circle around (5)} Investment and maintenance costs are high. The above shortcomings limit its wider application.
B. Flat-plate photobioreactor
Flat-plate photobioreactor has been described by Samson and Leduy (1985 years) for the first time. Now it mainly includes artificial light plate box-type photobioreactor, horizontal flat plate photobioreactor, vertical rabbet plate photobioreactor, multi-layer parallel-plate photobioreactor, oblique bubbling plate photobioreactor etc. Compared with the tubular system, the design of the flat-plate type has the following advantages: {circle around (1)} large light specific surface area, without “dark area”, improving the productivity of photosynthesis; {circle around (2)} generally use the ventilation means to promote the mixture and turbulence, low power consumption by air-lift circulation, small damage of shear stress on cells, good mixing, heat transfer and mass transfer efficiency; {circle around (3)} timely release of oxygen relatively which can be greatly reduced the inhibition of microalgae growth by molecular oxygen; {circle around (4)} cleaning and maintaining relatively simple. Therefore, the flat-plate reactor as a basic structure has certain advantages.
The structure of plate box type reactor is simple, but it also has some disadvantages, mainly: {circle around (1)} light supply and utilization efficiency. Illumination is one of the most important factors limiting reactor amplification and high-density cultivation of microalgae. When using the external natural light, most patents did not consider how to effectively enhance the algal mixture in the direction of illumination. Coupled with the curtaining of light by surface microalgae, the utilization of light and photosynthesis are limited, which leads to a smaller scale of the reactor. Therefore, there are many patents mainly taking advantage of the internal light source in order to expand the size of the reactor, which increases the operating costs, such as box-type photobioreactor (U.S. patent. US005104803A, Jan. 14, 1992) and internal-light box-type photobioreactor (U.S. Patent. US2003/0059932A1, Mar. 3, 2003). Other scholars (such as Jorg Degen, Li Guang, etc.) add a variety of internal baffle or deflector to achieve enhanced mixing effect of light direction, but it also makes the internal manufacturing and cleaning difficult. Therefore, how to combine the algal concentration, light intensity distribution and mixed-mode to optimize the structure and composition of the reactor in order to enhance solar energy (especially natural light) use efficiency is the core of designing photobioreactor; {circle around (2)} gas supply problems. Because high CO2 concentrations is harmful to microalgae, basically the existing technology use the air to dilute the CO2 to a extent concentration and then send the mixture into the photobioreactor by gas supply device, and simultaneously meet the mixing and mass transfer of CO2 in the algae liquid and lifting the algae liquid. But this also causes difficulties: when the gas supply device adopts small aperture to enhanced CO2 mass transfer, and the mass transfer of oxygen is also enhanced, it allows the power consumption to increase. On the contrary, large aperture devices will limit the absorption and utilization of CO2. Basically, the existing technology in order to dilute the CO2 to a certain concentration in the air after the air supply unit into the light through the bioreactor, and at the same time meet the CO2 in the algae fluid mixing, mass transfer, and enhance the role of algal fluid,
As the existing flat-plate reactor is difficult to amplify, the largest volume of the reactor unit is less than 300 L. Large-scale culture systems are realized by increasing the reactor units, which greatly increases manufacturing costs. Excessive reliance on internal sources also significantly increases operating costs. Therefore, the flat-plate reactor, more often at an experimental stage, has not been applied to the industrialization of algae cultivation.
C. Fermenter
Use traditional fermentation tank for microalgae heterotrophic cultivation without restriction of light, rapid growth speed, high yield per unit volume, biomass greatly increased. Using more mature industrial fermentation technology like fed-batch technology is expected to achieve high-density culture, to facilitate reactor amplification and well automated control, but is currently limited to cultivation of a few species of microalgae.
From the foregoing, it is not difficult to find that the existing microalgae cultivation either using photobioreactor for photoautotrophic cultivation with slow growth of microalgae or using fermentation tank for heterotrophic cultivation utilizing organics. Through the appropriate proportion of light-dark area designed and the dispensing of different carbon sources within the photobioreactor, microalgae photoautotrophic process and heterotrophic process can be coupled in one reactor and thus better to play the strong points of photoautotrophic and heterotrophic cultivation, which favors photobioreactor amplification and microalgae cultivation with high-density and low cost, and simultaneously can achieve the goal of organic wastewater utilization and carbon mitigation for wastewater treatment.